With recent advances in the digital technology, it has become popular to perform digital processing for images even in the medical field. A preferred application example of digital image processing is DSA processing of obtaining digital subtraction angiography (to be referred to as DSA hereinafter). A DSA image is an image obtained by obtaining images before and after the inflow of a contrast medium into an object, and subtracting an image (mask image) before the inflow of the contrast medium from an image (live image) after the inflow of the contrast medium. In the inter-image difference processing of subtracting the mask image from the live image, a vascular region serving as a region of interest in diagnosis is obtained as a change region between the images that is generated by the inflow of the contrast medium. Furthermore, in the inter-image difference processing, a region except for the vascular region serving as a region of interest is removed as a background region. The thus generated DSA image is an image useful for diagnosis because the vascular region serving as a region of interest can be observed without any influence of an object structure.
The purpose of using a DSA image for diagnosis is clear depiction of a contrasted vascular image. This purpose is achieved by subtracting a mask image from a live image. Since, however, an object generally moves, a motion artifact unwantedly appears in a DSA image. In a DSA image expected to ideally depict only a vascular region into which a contrast medium has flowed, a motion artifact interferes with clear depiction of a vascular image. Especially if a motion artifact occurs in a vascular region, angiostenosis or the like becomes very difficult to see.
To solve this problem, PTL 1 discloses a method of setting, particularly based on a motion artifact, a region of interest in a subtraction image obtained by subtracting a mask image from a live image, and performing registration by pixel shifting within the region of interest. Registration by pixel shifting is a method of reducing a motion artifact by shifting, upward, downward, rightward, or leftward, pixels to be subtracted when performing inter-image difference processing between a live image and a mask image. In this method, a motion artifact is automatically, effectively reduced by calculating a pixel shift amount from a region of interest set based on the motion artifact.